Biaxially oriented films based on crystallizable thermoplastics are known and widely described. Production of almost all biaxially oriented films uses various proportions of inherent recycled material, and this inherent recycled material is generally produced from cut film inherently produced by the process. Especially in the case of film thicknesses below 4 μm, and also for films of thicknesses more than 300 μm, the use of large proportions of inherent recycled material is critical, because the production of these films often becomes unreliable. In the thickness range from 5 to 300 μm, the proportion of inherent recycled material is generally from 20 to 40%, based on the total weight of the thermoplastics.
In the case of very thin and very thick films, i.e. films which are produced with a proportion of inherent recycled material that is usually low, yields during film production are rather low, and this results in an excess of cut film and therefore of inherent recycled material. This sometimes makes the production of these films uneconomic.
EP-A 0 483 665 discloses a process which uses melt post-condensation to increase molar mass in the treatment of waste polyester materials with various molar masses and finishing specifications. However, the advice given is to avoid using treated polyester in biaxially oriented films which have the highest finishing specification.
EP-A 1 418 195 describes a process for production of a thermoplastic film using recycled material from plastics bottles. The use of these secondary raw materials (recycled material from plastics bottles) is in particular impossible in the case of films in medical applications (lack of traceability) and is subject to limitation in the case of white films. Furthermore, the use of these recycled materials in films intended for food-and-drink applications is not approved in some countries. The recycled materials or flakes based on PET bottles also often comprise other additives, such as isophthalic acid, which can impair the mechanical properties of the films. The use of these secondary raw materials is therefore subject to a limit of at most 50% by weight.